The present invention relates to a Raman amplified optical system and, more particularly to the utilization of a transmission fiber having predetermined dispersion characteristics so as to reduce the presence of modulation instability and four-wave mixing effects.
The subject of Raman amplification is well known in the literature. Stimulated Raman amplification is a nonlinear optical process in which an intense pump wave is injected into an optical fiber that is carrying one or more optical signals. In fused silica fibers, if the pump wavelength is approximately 100 nm shorter than the signal wavelength in the vicinity of 1500 nm, the pump will amplify the signal(s) via stimulated Raman scattering. If the amplification is made to occur in the transmission fiber itself, the amplifier is referred to as a xe2x80x9cdistributedxe2x80x9d amplifier. Such distributed amplification has been found to improve the performance of a communication system, as discussed in the article xe2x80x9cCapacity upgrades of transmission systems by Raman amplificationxe2x80x9d, P. B. Hansen et al, appearing in IEEE Phot. Tech. Lett., Vol. 9, 1997, at page 262. For example, if a pump wave is injected into one end of the fiber in a direction counter-propagating with respect to the information signals, the signals will be amplified before their signal-to-noise ratio degrades to an unacceptable level. The performance of such an amplifier is often characterized in terms of its effective or equivalent noise figure and its on/off gain. The effective noise figure is defined as the noise figure that an equivalent post-amplifier would have in order to achieve the same noise performance as the distributed Raman amplifier (see, for example, xe2x80x9cRayleigh scattering limitations in distributed Raman pre-amplifiersxe2x80x9d, by P. B. Hansen et al., IEEE Phot. Tech. Lett., Vol. 10, 1998 at page 159). Experimentally, the effective noise figure may be found by measuring the noise figure of a span utilizing counter-propagating Raman amplification and then subtracting (in decibels) the passive loss of the span. The on/off gain of a distributed Raman amplifier is defined as the difference (in decibels) between the output signal power with the Raman pump xe2x80x9conxe2x80x9d to that with the pump xe2x80x9coffxe2x80x9d.
The concepts of group velocity and group-velocity dispersion are well known in the field of fiber optics. Group velocity is defined as the velocity at which an optical pulse will travel, while group-velocity dispersion is defined as the change in group velocity as a function of wavelength. The group-velocity dispersion, D, is often characterized in terms of ps/nm-km. In these terms, therefore, if light is traveling in an optical waveguide (such as an optical fiber), the group-velocity dispersion depends not only on the materials from which the waveguide is fabricated, but also on the specific design of the index structure used to guide the light. The latter contribution, known as waveguide dispersion, can be used to significantly alter the dispersion characteristics of optical fibers. A complete discussion of this topic can be found in Fiber-Optic Communication Systems, Agrawal, John Wiley and Sons, Inc., 1992, at Chapter 2.
The dispersion characteristics of an optical fiber are often be characterized by its zero-dispersion wavelength (ZDW)xe2x80x94the wavelength at which the group velocity dispersion is zero, and its dispersion slopexe2x80x94the change in group velocity dispersion as a function of wavelength. For example, standard single-mode optical fiber (SSMF) has a dispersion that is dominated by the material dispersion of the fused silica and therefore has a ZDW of approximately 1300 nm and a dispersion slope of 0.07 ps/nm2-km.
It is also well known that the effects of group velocity dispersion can be deleterious to the performance of optical communication systems, particularly those employing Raman amplification. For example, in a communication system employing on/off keying, group-velocity dispersion may cause pulses to broaden, extending pulses into their neighboring bit slots and thus introducing errors into the transmitted information signal. Although this effect can be ameliorated by the inclusion of dispersion-compensating devices located periodically throughout the communication system (but at an additional expense) it is advantageous to keep the dispersion of the transmission fiber below 10 ps/nm-km.
Another property of an optical fiber in an optical transmission system which must be controlled is the effective area at the signal wavelengths (see Nonlinear Fiber Optics, Agrawal, Academic Press, 1995, second edition, pg. 43, Eq. 2.3.29 for more of a description of xe2x80x9ceffective areaxe2x80x9d). If the effective area is increased, then the distributed Raman amplification in the fiber becomes less efficient. However, if the effective area of the fiber becomes too small, then other nonlinear optical effects become larger and degrade the performance of the optical transmission system. Therefore, the transmission fiber must have an effective area that balances the efficiency of the distributed Raman amplification and the degradation of the system from other nonlinear effects.
In the early 1990""s, experiments were performed on the transmission of information on multiple wavelengths within a single optical fiber. It was found that a nonlinear optical interaction known as xe2x80x9cfour-wave mixingxe2x80x9d (FWM) (also referred to in the art as four-photon mixing) could limit the performance of the communication system. In FWM, three frequencies, denoted xcexdi, xcexdj and xcexdk (xcexdkxe2x89xa0xcexdi, xcexdj), interact through the fiber nonlinearity to generate a new frequency, xcexdijk=xcexdi+xcexdjxe2x88x92xcexdk. Since Raman amplified systems utilize an information signal propagating at the signal wavelength, and separate strong pump signals (comprised of multimode pump lasers or several single mode pump lasers) at different pump wavelengths, FWM can occur. The concept of four-wave mixing is well known in the literature, and is discussed in detail in the reference Optical Fiber Communications, IIIA, Kaminow and Koch, Academic Press, San Diego, 1997, at chapter 8. It is known that the strength of four-wave mixing can be significantly decreased by increasing the fiber dispersion at the mixing wavelengths. A new class of optical fibers, known as non-zero dispersion-shifted fibers (NZ-DSF) and disclosed in U.S. Pat. No. 5,327,516 issued to A. R. Chraplyvy et al., shift the ZDW of the fibers away from 1550 nm to slightly higher or lower wavelengths, thus adding a small amount of dispersion at those wavelengths. However, current types of NZ-DSF have dispersion zeroes in the wavelength range of 1480-1510 nm.
Another nonlinear optical process well known in the prior art is modulation instability. In this nonlinear optical process, the nonlinear refractive index serves to phase match a four-wave mixing process that would otherwise have been phase mismatched. The result is the generation of sidebands about the injected wavelength for small, positive values of group-velocity dispersion (D), where the frequency offset of the sidebands increases with decreasing dispersion.
Since it will be desirable, in future systems, to use a relatively high power Raman source, as well as multiple Raman pump sources, it is necessary to develop an arrangement for avoiding the effects of modulation instability and four-wave mixing in a Raman amplified optical transmission system.
The need remaining in the prior art is addressed by the present invention, which relates to a Raman amplified optical system and, more particularly, to a Raman amplified optical system using a transmission fiber having predetermined dispersion characteristics so as to reduce the presence of modulation instability and four-wave mixing effects.
In accordance with the present invention, a Raman amplified optical system is formed to include a transmission fiber defined by a pre-determined set of constraints so as to limit the above-described effects. In particular, a transmission fiber of the present invention is designed to limit modulation instability by exhibiting either a non-positive dispersion or a dispersion greater than +1.5 ps/nm-km at any desired pump wavelength, while also exhibiting a dispersion with a magnitude less than 10 ps/nm-km at a signal wavelength. It is has been found that by carefully controlling the dispersion (D) to remain in these regions, any continuum (i.e., sidebands) generated by the pump will remain relatively narrow and away from regions of large Raman amplification. Thus, any noise components associated with this continuum will be minimal.
Additionally, and further in accordance with the present invention, the presence of four-wave mixing (FWM) is reduced by ensuring that the zero-dispersion wavelength (ZDW) of the transmission fiber is not centered between the pump wavelength and any signal wavelength experiencing large Raman gain. It has been found that if the ZDW is approximately the average of these two wavelength values, the dispersion of the fiber will allow phase matching of FWM components having frequencies within the signal band, allowing these FWM noise components to be amplified and reducing the quality of the transmitted signal.
Other and further characteristics of a Raman amplifier transmission fiber and communication system formed in accordance with the present invention will be apparent during the course of the following discussion and by reference to the accompanying drawings.